Fourier transform infrared study on black soap films

Abstract

Fourier transform infrared (FT-IR) spectra in the range 3800–2800 cm−1 have been recorded of the first and second black films in air prepared from aqueous solutions of sodium dodecylsulfate (SDS) without and with the addition of sodium chloride, respectively. It is found that in both films the methylene chains of SDS, which are located outside the aqueous core of the sandwich structure, are in an oriented liquid crystalline state. In a first black film, the thickness of the aqueous core is determined to be 9.6 nm, by recording polarized infrared spectra with various angles of incidence to the film surface, and by analyzing them with the aid of thin layer optics. It is found that this value varies in the range 5 ~ 10 nm from film to film. The extinction coefficients of the aqueous core are also examined. In the second black film, the thickness of the aqueous core is found to be about 1 nm. The OH stretching band of water in the second black film appears at appreciably higher wavenumbers (by 60 cm−1) than those of the first black film, indicating a high ionic strength in the aqueous core. In both black films, there is no sign of molecular orientation or rigidified structure of water.